This invention relates to a drive shield for use in driving tunnels. Throughout this specification, the term "tunneL" or "tunnels" should be taken to include mine galleries, adits, trenches or other elongate excavations.
A known type of tunnel drive shield consists (see DT-AS No. 2,021,734) of a plurality of elongate planks (knives) which are arranged side-by-side and parallel to the axis of the tunnel. The planks define a generally semi-cylindrical shield and are supported by two axially-spaced frames. The planks can be advanced, either individually or in groups, by means of hydraulic rams which abut one of the frames. The frames are connected to control elements disposed along the tunnel, and are movable relative to one another in the direction of tunnel advance.
Each frame can be extended vertically so that it can be braced directly against the tunnel floor and indirectly (by way of the planks) against the tunnel walls and ceiling. During advance, the rear support frame is anchored and the front, unbraced frame pulled forward by the control elements. Then, the front frame is anchored and the rear frame is released and pulled forward by the control elements. Thus, one support frame is always under load and constitutes an abutment for the other frame. This type of drive shield suffers from a considerable defect, namely that, during advance, one of the frames (in particular the front frame) may be displaced either vertically or transversely, this displacement arising from play between the frames and the planks. This problem is exacerbated by rock and earth jamming against the advancing parts and influencing the direction of advance. Moreover, particularly in soft earth, the front frame may sag somewhat during its advance movement. Basically, therefore, this type of drive shield cannot be advanced accurately in the required direction.
The main object of the invention is to provide a tunnel drive shield that can cut through earth and rock, particularly in zones of differing compositions, along a path that coincides as closely as possible with that intended.